1. Field of the Invention
Embodiments of the present invention relate to the field of radiography using tomosynthesis, and more particularly the field of methods for assisted positioning of an organ, e.g. a breast, for a radiography session by tomosynthesis.
2. Description of the Prior Art
Mammography is conventionally two-dimensional radiography. The patient's breast is positioned on a breast platform with respect to a harmless radiation light source, and then compressed by a compression paddle. The breast is positioned so that it is entirely irradiated by the rays of the X-ray source, i.e. an imaged field illuminated by the light source is the same as the field illuminated by the X-ray source.
For this purpose, when positioning the breast, the imaged field and the breast are illuminated by a light source. Since the light source and the X-ray source cannot physically occupy the same position, a mirror is provided to deflect the light source so that it virtually merges with the X-ray source and illuminates the same imaged field as the X-ray source. It is therefore easy to know which part of the breast will be illuminated by the X-ray source.
Another type of mammary radiography also exists, allowing a three-dimensional image of this organ to be obtained: mammary radiography by tomosynthesis.
With mammary radiography by tomosynthesis, several images of a breast, which is held in position, are acquired at different positions of an X-ray source of an acquisition system with respect to a detector. Usually, the breast is positioned on a breast platform in which the detector of the acquisition system is arranged. The breast is then compressed by a compression paddle. Several images are then acquired with the source moving from a starting position to a finishing position; the breast, the platform and the paddle remain in position. The source describes a movement with respect to the detector. This movement is generally a rotation about a point located on a plane passing through the breast, in the center of the edge of the detector lying opposite the patient.
A 3D image of the breast is then reconstructed from the acquired images. The quality of reconstruction depends upon the beam angle (angle between the two end positions of the source) and the number of acquired images.
Contrary to conventional 2D mammography, with breast radiography by tomosynthesis, several images of the breast are taken at different positions of the X-ray source. It is therefore not possible to know which parts of the breast will be irradiated by the X-ray source at all the positions thereof. At each of its positions, the X-ray source effectively illuminates a different portion of the space between the source and the detector.
Therefore, if the positions of the breast are simply controlled in the same way as for conventional 2D mammography, it will only be possible to position the breast correctly for one single position of the X-ray source.
In addition, the possible illumination of the entire breast also depends on the breast thickness.
FIGS. 1 and 2 illustrate the positioning of breasts O1 and O2 having a different thickness when they are compressed by the compression paddle 26, and the portion of space illuminated by the source 24 at each of the successive positions S1-S9 thereof (here, as an example, nine positions are illustrated but there may be a different number of positions).
For reasons related to health safety, the illumination cone produced by the source 21 must not project too far beyond the detector 251 to avoid unnecessary patient irradiation.
FIG. 1 shows a compressed breast O1 of small thickness, typically of 3 cm. It is noted that, irrespective of the position of the source 21, this compressed breast O1 is entirely illuminated. Therefore, each of the acquired images contains information on the entirety of the breast O1. The reconstructed 3D image will thus have a good quality and will be reliable.
FIG. 2 illustrates a compressed breast O2 of large thickness, e.g. 12 cm, whose width is identical to that of the compressed breast O1 of small thickness in FIG. 3. During the acquisition of images, at some source positions (solid line—S1-S3 and S7-S9), parts PO2 of the breast O2 are projected outside the detector 251. At these positions, the image acquired by the detector 251 does not contain any information on these non-irradiated parts PO2. During three-dimensional reconstruction (3D) of the breast O2, the lack of information in these images creates artefacts. The volume reconstruction of the breast does not give a true representation thereof.
This problem particularly occurs during radiography in cranio-caudal mode (CC—the detector lies parallel to the floor) with respect to the two side lobes of the breast, and in mediolateral-oblique mode (MLO—the detector lies 45° to the vertical) with respect to the lower breast lobe.
At the present time, no method is available to remedy the lack of information on a part of the breast in the acquired images.